As a conventional position determination system, a GPS (Global Positioning System) is known. A satellite for emitting a signal for use in the GPS (the satellite and the signal will hereinafter be referred to respectively as “GPS satellite” and “GPS signal”) is flying at an attitude of about twenty thousand km from the ground. Any user is allowed to receive the signal emitted from the GPS satellite so as to measure a distance between the GPS satellite and the user through demodulation of the received signal. Thus, as long as there is no obstacle between the ground and the GPS satellite, position determination can be performed using the signal emitted from the GPS satellite. However, in cases where the GPS is used, for example, in an urban area, a forest of buildings often becomes obstacle to cause a situation where a position information providing apparatus of a user cannot receive the signal emitted from the GPS satellite. Further, depending on conditions, a signal diffraction or reflection often occurs due to the buildings to cause an error in distance measurement using the signal, which leads to deterioration in accuracy of position determination.
Although there is a technique of receiving in an indoor area a weak GPS signal which has penetrated through a wall or roof, a receiving state is still unstable, which causes deterioration in accuracy of position determination.
While the above description has been made about position determination by taking the GPS as an example, the aforementioned phenomena are observed in a satellite-based positioning system in general. As used herein, the term “satellite-based positioning system” means any type of satellite-based positioning system including GLONASS (GLObal NAvigation Satellite System) in Russian Federation, and Galileo in Europe, as well as the GPS.
A technique related to a position information providing system is disclosed, for example, in JP 2006-67086A (Patent Document 1).
However, in a technique disclosed in the JP 2006-67086A, there is a problem that it lacks versatility, because a reader or a writer is unique as an element of a position information providing system. Moreover, due to a need for limiting a transmitter output to avoid interference, a receivable range for position information is restricted, which causes a problem that it is unable to continuously acquire position information, or an extremely large number of transmitters are required to cover a wide range.
In a conventional mobile phone having a position determination function, position information can be acquired in a location where it is possible to receive a signal from a satellite, so that a position of the mobile phone can be notified. However, in a location where it is impossible to receive radio waves, such as an inside area of a building or an underground shopping area, there is a problem of being unable to acquire position information.
As measures for this problem, for example, a technique is contemplated which is designed to arrange in an indoor area a plurality of transmitters each capable of emitting a signal similar to a GPS signal so as to calculate a position based on the trilateration principle as in the GPS. However, in this case, there is a problem of an increase in cost of the transmitters due to a need for allowing respective clock times of the transmitters to be in synchronization with each other.
Moreover, a radio wave propagation becomes complicated due to reflection in the indoor area, which causes another problem that an error of about several tens of meters easily occurs despite the installation of such costly transmitters.
JP 2007-278756A (Patent Document 2) discloses a position information providing system capable of seamlessly acquiring position information in indoor and outdoor areas, wherein position information (latitude, longitude, altitude, etc.) itself about an installation location of an indoor transmitter is transmitted in the same format as a conventional format of a signal from a GPS satellite, and a conventional configuration for position determination based on GPS satellites is used as hardware of a GPS receiver without modification.    Patent Document 1: JP 2006-67086A    Patent Document 2: JP 2007-278756A